Hyperthermia has been used for treatment of cancers for many years. It is known that raising temperatures of cells to above about 43.degree. to 45.degree. for sufficient time causes cell death and temperatures below about 41.5.degree. generally do not affect the cells. Some types of malignant cells reportedly can be destroyed by raising their temperatures to levels slightly below those injurious to most normal cells. One of the techniques which has been used for hyperthermia is heating of the blood of a patient by an external apparatus, thereby raising the temperature of the entire body or of a portion thereof to the therapeutic temperature. This procedure risks substantial injury to the patient if temperature is not carefully controlled, and may fail to raise the temperature of the malignant cells sufficiently for destruction. Any malignant cells which remain undestroyed may cause a recurrence of the tumor, growth or malignance (hereinafter tumor).
Some surface tumors may be successfully treated by application of surface heat from a heated object. Deeply located tumors, however, are difficult to heat to therapeutic temperatures without destruction of the overlying tissue.
Another hyperthermia technique uses electromagnetic radiation to heat tissue. The electromagnetic radiation is often in the form of radio frequency (RF) or microwave radiation because of the ease of generating, controlling and directing microwaves, and also because of the absorption characteristics of tissue at microwave frequencies. At the current state of the art, electromagnetic hyperthermia is usually at frequencies in the range of 10 MHz to 6 GHz (herein termed radio frequency and microwave or RF&M). Radio frequency and microwave hyperthermia when applied to tissue containing a tumor generates heat within the tissue which raises the temperature of the tissue generally. It has been found that most tumors tend to have a limited blood supply by comparison with healthy tissue. Thus, the circulation of blood through a tumor is usually low by comparison with circulation through healthy tissue. At any electromagnetic power density, the tumor will usually be hotter than the surrounding healthy tissue because the more ample flow of blood in the healthy tissue provides cooling of the healthy tissue. Thus the tumor may be heated by RF&M hyperthermia to a therapeutic temperature without significant effect on the surrounding healthy tissue.
It has been found that RF&M hyperthermia when used in conjunction with either radiotherapy (ionizing radiation therapy) or with chemotherapy provides more consistent success that either alone. A course of treatment may include several radiotherapy treatments each week, interspersed with RF&M hyperthermia treatments. Widespread practical application of such combined therapy depends upon the availability of convenient and predictable RF&M hyperthermia method and apparatus.
U.S. Pat. No. 4,448,198 issued May 15, 1984, to Turner describes an invasive hyperthermia arrangement in which a plurality of microwave applicators are inserted into body tissue. The surgical implementation requires the use of an expensive operating room and the services of a skilled surgeon, which is not convenient. The applicators provide numerous potential sites for infection and at least require care by the patient. The implanted applicators may interfere with concurrent radiotherapy. Since the dielectric constant of the tumor may differ somewhat from that of the surrounding tissue, the energy from the microwave applicators may be partially reflected by the tumor if the applicators are implanted in healthy adjacent tissue, and this may result in an undesirable temperature distribution.
Noninvasive radio frequency and microwave hyperthermia relies upon heating from applicators placed outside the patient's body. This is particularly convenient for small surface tumors, the extent of which can be readily seen. The applicator is often held in contact with the surface being treated to avoid excessive spreading of the energy. The center of the applicator is directed towards the tumor, and the power is applied. Adjacent normal tissue is likely to be at a lower temperature than the temperature at the tumor because a simple applicator such as a horn has a power distribution which decreases away from the center or axis.
Tumors deep within tissue are more difficult to treat with radio frequency or microwaves, since the tissue overlying the tumor absorbs energy from the electromagnetic field entering the body. Thus, the electromagnetic field density at progressively deeper levels of the tissue progressively decreases. The increase in tissue temperature due to microwave heating tends to be a maximum at the surface of the tissue and to progressively decrease with increasing depth for uniform tissue heated by an applicator such as a horn antenna. Naturally, the actual amount of heating depends upon the relative absorption of the various different layers of tissue as a function of depth, the quantity and distribution of blood vessels available for cooling the various layers of tissue, and surface effects such as cooling of the skin by perspiration or air circulation.
A deeply located tumor can be heated to necrotic temperatures with an applicator such as a horn antenna, but this results in a power density at the surface of the skin sufficiently high to create burned areas. Burned areas subjected to radiotherapy tend to heal slowly or not at all. Ordinarily, radiotherapy is discontinued if the area to be irradiated is injured. Any burning of a part of a tumor or of the overlying tissue is therefore undesirable, as it limits therapeutic options. Thus, the treatment of deeply located tumors by noninvasive radio frequency or microwave techniques is difficult.